Central multilevel current-fed inverter with module integrated DC-DC converters for grid-connected PV plant

This paper proposes a system of current-fed converters for the integration of a large photovoltaic (PV) plant with a utility ac grid. The system comprises a large number of PV modules with module integrated dc-dc converter. A dual inductor current-fed push-pull topology is proposed for this purpose. To meet the THD requirement along with low switching losses a central multilevel current-fed inverter (CFI) is proposed. It comprises two three-phase three-level CFI units connected in parallel on the ac side. The dc-dc converters inject continuous current into the dc-bus of the CFI units. Five-level output current waveform is obtained using space vector modulation (SVM) technique when dc-bus currents are equal. However, mismatch in module characteristics and non-uniform insolation levels cause unequal dc-bus currents leading to distortion of the output current waveform. This limitation is addressed by appropriate selection of the switching state vectors while employing SVM technique. Two methods for selection of the switching state vectors are presented. The performance of a 100 kW PV plant utilizing the proposed power converters system is studied using MATLAB/Simulink.